Mojave Security update seems in a loop
I've got a late 2012 iMac which I recently updated from High Sierra to Mojave. Semed like it was going to be okay until I saw that I needed to run a security update. which was about 8 hours ago. Since then, all I've seen on the screen is the apple logo (no progress bar) which stays on for maybe 5-10 minutes, and then the screen goes black. This just repeats that process. Seems like it's trapped in that loop. Is there anything I can do? I've got a time machine backup drive so I'm of the thinking that if I need to just reformat and start over, I can do that. Reinstalling applications will be painful but may help me purge what I don't need, too.
What are my options at this point?
iMac 27″, macOS 10.13